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PT Notes

Worst-Case Scenarios in Process Hazard Analysis (PHA)

PT Notes is a series of topical technical notes on process safety provided periodically by Primatech for your benefit. Please feel free to provide feedback.

Process safety regulations in the US require that PHA studies address the consequences of failure of engineering and administrative controls, i.e. process safeguards. A range of consequences usually is possible for a hazard scenario depending on which, if any, safeguards fail. The scenario variants differ by degree of damage or injury. It is not feasible to analyze each variant so practitioners must select a representative scenario from the set involving combinations of safeguard failures. The most logical choice is the variant in which all safeguards fail, which produces the worst-case consequence.

Thus, assignment of scenario severity values usually is based on a worst-case evaluation of the consequence, i.e., all safeguards are assumed to fail. Note that this is not the same as assuming there are no safeguards present. The presence of safeguards is taken into account by incorporating their failure probabilities into the scenario likelihood estimate. This compensates for the assumption of worst-case consequences for the severity estimate.

Some practitioners take credit for the successful operation of passive safeguards which generally are more reliable than active safeguards. Passive safeguards are systems that are not physically actuated in order to perform their intended functions, e.g. a blast wall. Active safeguards are physically actuated, e.g. a deluge system.

Note that the worst-case consequence scenario is not necessarily the worst-case risk scenario. This issue has been addressed in:

Process Hazards Analysis, Handbook of Loss Prevention Engineering, Wiley-VCH, 2013.

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